Photosensors made of semiconductor materials have gained attention in recent years because they may be specially adapted for the needs of many optical applications. See D. Renker, Properties of avalanche photodiodes for applications in high energy physics, astrophysics and medical imaging, Nuclear Instrumentation and Methods A, 486, pp. 164-169, (2002); S. Vasile, P. Gothoskar, R. Farrell and D. Sdrulla, Photon detection with high gain avalanche photodiode arrays, IEEE Transactions on Nuclear Science, 45(3), pp. 720-723, (1998); B. F. Aull, A. H. Loomis, D. J. Young, R. M. Heinrichs, B. J. Felton, P. J. Daniels, and D. J. Landers, Geiger-Mode Avalanche Photodiodes for Three-Dimensional Imaging, Lincoln Laboratory Journal, 13(2), pp. 335-350, (2002); J. C. Jackson, D. Phelan, A. P. Morrison, R. M. Redfern, and A. Mathewson, Towards integrated single photon counting microarrays, Optical Engineering. 42(1), pp. 112-118, (2003); C. Niclass, A. Rochas, P.-A. Besse, and E. Charbon, Design and characterization of a CMOS 3-D image sensor based on single photon avalanche diodes, IEEE Journal of Solid-State Circuits, 40(9), pp. 1847-1854, (2005); F. Zappa, A. Gulinatti, P. Maccagnani, S. Tisa, and S. Cova, SPADA: Single Photon Avalanche Arrays, IEEE Photonics Technology Letters, 17(3) pp. 657-659, (2005); E. Sciacca, S. Lombardo, M. Mazzillo, G. Condorelli, D. Sanfilippo, A. Contissa, M. Belluso, F. Torrisi, S. Billotta, A. Campisi, L. Cosentino, A. Piazza, G. Fallica, P. Finocchiaro, F. Musumeci, S. Privitera, S. Tudisco, G. Bonanno, and E. Rimini, Arrays of Geiger mode Avalanche Photodiodes, IEEE Photonics Technology Letters, 18(15), pp. 1633-1635, (2006); and F. Zappa, S. Tisa, A. Tosi and S. Cova, Principles and features of single photon avalanche diode arrays, Sensors and Actuators A 140, pp. 103-112. (2007).
Due to their properties, photon counting detectors have found widespread use for the detection of very weak and fast optical signals in many fields like astronomy, fluorescence and luminescence decay measurements, single-molecule detection and laser ranging. (M. Ghioni, S. Cova, F. Zappa, and C. Amori, Compact active quenching circuit for fast photon counting with avalanche photodiodes, Review Scientific Instruments, 67(10), pp. 3440-3448, (1996)).
Silicon photomultipliers (SiPM) are semiconductor photodetectors operated in a limited Geiger mode. The SiPM structure is based on a two-dimensional pixel array of Geiger mode Avalanche Photodiodes (GM-APD) that individually act as photon counters. In Geiger mode, each pixel (photodiode) is operated with a bias voltage above its breakdown voltage. At this operating condition, the electric field within the GM-APD depletion layer is high enough that charge carriers injected in this region may trigger a self-sustaining avalanche multiplication process by impact ionization mechanisms.
When a photon is absorbed in the depletion layer of the diode, a photo electron-hole pair is generated. This pair may initiate the breakdown current. As a result, a sharp current pulse of few milliamps and with sub-nanosecond rise time is produced. The current rising edge marks the photon arrival time. In Geiger mode, photodiodes work as binary devices, because they give the same output signal regardless of the number of interacting photons. (E. Sciacca, A. C. Giudice, D. Sanfilippo, F. Zappa, S. Lombardo, R. Cosentino, C. Di Franco, M. Ghioni, G. Fallica, G. Bonanno, S. Cova, and E. Rimini, Silicon planar technology for single-photon optical detectors, IEEE Transactions on Electron Devices, 50(4), pp. 918-925, (2003)).
In a SiPM, the current flow throughout each pixel (photodiode) is limited by integrated individual quenching resistors, which turn off the avalanche current and reset each fired diode. In SiPM configuration, each diode is connected to the power supply through an integrated large series ballast resistor Rq (typical values in the range 100 kΩ-3MΩ). (P. Buzhan, B. Dolgoshein, L. Filatov, A. Ilyin, V. Kantzerov, V. Kaplin, A. Karakash, F. Kayumov, S. Klemin, E. Popova, and S. Smirnov, Silicon photomultiplier and its possible applications, Nuclear Instrumentation and Methods A, 504, pp. 48-52, (2003)).
In a SiPM, the outputs of all the pixels are multiplexed; therefore, it is not possible to localize the pixels fired by the absorption of photons. This drawback may limit the performance of this device and make its use possible only in applications where it is important only to determine the number of photons impinging on the detector surface, regardless of their point of absorption.
Typically, to overcome this limitation in applications like Positron Emission Tomography (PET), several Photomultiplier Tubes (PMTs) or SiPMs are tiled together to form along with a two-dimensional array of scintillating crystals a unique block detector, as shown in FIG. 5. In PET applications, the crystals in the scintillator block are usually optically isolated from each other, and each scintillator is coupled to a SiPM with size comparable to the transverse dimensions of the coupled crystal.
When a gamma-photon is incident on one of the crystals, the resultant light may be conveyed within the same crystal if the scintillators in the block are optically isolated or shared by the different scintillators otherwise. Information on the position of the detecting crystal may be obtained from the detector outputs by using complex position reconstruction algorithms if the light emitted by the scintillators is shared among different detectors, as it is typical of tiled arrays coupled to scintillators that are not optically isolated. On the other hand, also in the case where the scintillators are isolated and coupled to independent detectors, it is helpful to have independent anode and cathode outputs for each one of the SiPMs within the array to determine the gamma-ray impinging position on the scintillator block. However, the spatial resolution allowed with this technique may be poor due the limited number of detectors that may be tiled together because of the complexity of the electronics, packaging constraints, the finite dimension of the same detectors required to obtain a measurable signal at their outputs, and the limited fill factor of the whole tiled array.
This requirement may also be important in imaging arrays using GM-APD pixels as sensitive elements to exploit fast response of these detectors in LIDAR techniques. To provide adequate spatial information without using readout scanning techniques, individual outputs with appropriate driving and readout circuits for each pixel of the array may be required. Obviously the larger the number of pixels in the array, the higher its dynamic range, and consequently, the more accurate the spatial information provided by the whole photodetector. Moreover the higher the whole dimension and the geometrical fill factor of the array, the higher its sensitivity. However, because of packaging constraints and the reduction of the fill factor due to the individual pixels outputs, arrays with a reduced number of pixels (typically 1000) may only be realized by using this approach.